Bully-Proofing Your School (BPYS), a school-based intervention program designed to reduce bullying and school violence, is evaluated for its impact on bullying and related aggressive behaviors in a multiple nonequivalent control group, pretest-posttest design with ex ante selection of treatment and comparison groups. Outcome measures included perceptions of school safety, perceived discouragement of aggression at school, and physical and relational aggression victimization and perpetration. Over a span of 5 years (1 pretest year, 3 intervention years, and 1 postintervention year) a total of 3,497 students were surveyed. The results suggest that BPYS led to (a) students recognizing that bullying was being discouraged in the school, (b) reduced bullying and related behaviors, and (c) weakly, perceptions of increased safety at school. These results tended to become weaker once schools implemented the program without technical assistance from program staff.